JOURNAL GEOLOGICAL SOCIETY OF INDIA
Vol.72, August 2008, pp.263-277
Mineralogy and Geochemistry of the Bengal Anorthosite Massif
in the Chotanagpur Gneissic Complex at the
Eastern Indian Shield Margin
NARESH C. GHOSE1*, NILANJAN CHATTERJEE2, DIPANKAR MUKHERJEE3,
RAY W. KENT4 and A. D. SAUNDERS5
1
Department of Geology, Patna University, Patna - 800 005, India
* G/608, Raheja Residency, Koramangala, Bangalore-560034
2
Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology,
Cambridge, Massachusetts 02139, U.S.A.
3
Geological Survey of India, Kankarbag, Patna - 800 020, India
4
Research Office, Rutland Hall, Loughborough University, Loughborough, Leicester LE1 3TU.
5
Department of Geology, Leicester University, Leicester LE1 7RH, U.K.
Email: ghosencprof@rediffmail.com
Abstract: The Bengal anorthosite is a narrow 40 km long, “tadpole-shaped” massif that occurs within granulite facies
rocks of the Proterozoic Chotanagpur Gneissic Complex at the eastern margin of the Indian shield. The core of the
massif consists of grey anorthosite with coarse-grained cumulus plagioclase megacrysts showing magmatic flow-related
alignment and the periphery consists of a mixture of the megacrysts and medium-grained, equigranular white anorthosite.
Repetitive graded layers of grey and white anorthosite and cyclic variation of elemental concentrations characterize the
massif at depths. These features are consistent with emplacement of the Bengal anorthosite through episodic magma
pulses. Labradorite (An57-58) is the major constituent with clinopyroxene (Mg# 62), hornblende (Mg# 36-44), ilmenite
and occasional orthopyroxene occurring as minor phases. Thermobarometric pressure-temperature estimates of the
anorthosites (4.1-7.3 kbar and 593-795oC) are similar to earlier studies achieved from the metabasic and gneissic country
rocks, and correspond with the last high-grade (Grenvillian) metamorphism. Similar Zr/Nb, Zr/Hf and Th/Ce ratios of
the anorthosites and oceanic island basalt possibly indicates derivation from a mantle source. Lower crustal interaction
is evident from similar Zr/Y, La/Nb and Th/Ce ratios of the anorthosites and lower continental crust. Anatectic upper
crustal melts probably contaminated the anorthosites as indicated from an enriched LILE pattern of the anorthosite.
Metabasic rocks associated with the anorthosites have lower crustal Zr/Y, Nb/Y and Zr/Nb ratios. Minor gabbroic
anorthosites within the massif, rich in iron and incompatible elements, were perhaps derived by differentiation of a
coeval mafic parental magma. Proximity of the anorthosite to the Damodar Graben indicates that the Bengal anorthosite
may have been emplaced in an extensional tectonic setting.
Keywords: Bengal anorthosite, Chotanagpur Gneissic Complex, Labradorite, Cyclic layers, Damodar Graben.
INTRODUCTION
Labradorite and/or andesine-rich Proterozoic anorthosite
massifs have been recognized and investigated in many
parts of the world (Ashwal, 1993; Wiebe, 1994).
Remarkably, most of them were emplaced between 1.0 and
1.8 Ga in a variety of low to high metamorphic grade
continental terrains. In spite of a voluminous literature on
massif-type anorthosites, the cause and mechanism of their
emplacement remains unsolved. Although high-pressure
phase equilibrium indicates a lower crustal origin (Green,
1969; Taylor et al. 1984; Longhi et al. 1999; Longhi, 2005),
it does not account for the large volume of magma required
to generate the large plutons. Many researchers prefer a
mantle plume-related origin and propose diapiric ascent of
plagioclase-rich crystal-mush derived by extensive
crystallization of the parental melts below the lower crust
(Emslie, 1985; Ashwal, 1993). Isotopic evidence suggests
that crustal contamination of the parental melts is rather
common (Emslie et al.1994; Wiebe,1994). Some field
evidence in the Nain plutonic suite suggests that the ascent
0016-7622/2008-72-2-263/$ 1.00 © GEOL. SOC. INDIA
264
NARESH C. GHOSE AND OTHERS
of the crystal-mush was aided by pre-existing faults and
fractures (Royse and Park, 2000). Metamorphism is common
in many massifs, but some massifs are undeformed (Emslie,
1978).
Proterozoic anorthosite massifs occur at the eastern
margin of the Indian shield in two mobile belts (Fig. 1A),
the Eastern Ghats Belt (EGB, Sarkar et al. 1981) and the
Chotanagpur Gneissic Complex (CGC, Mukherjee and
Ghose, 1992), where they are closely associated with highgrade amphibolite-granulite facies metamorphic rocks, sodic
alkaline complexes and granite plutons. The Bengal
anorthosite (Chatterjee, 1959) is an E-W elongated,
“tadpole-shaped”, 250 km2 large massif at the eastern
margin of the CGC. It is located between 23°27'48"N and
23°35'N latitudes and between 86°50'53"E and 87°15'E
longitudes at Saltora, West Bengal (Fig. 1). Based on new
mineralogical and geochemical data, the metamorphic
history, geochemical character, probable source and
emplacement mechanism of the Bengal anorthosite is
evaluated in this paper.
Table 1. Stratigraphic relationships of the Bengal anorthosite and
associated rocks
GEOLOGICAL SETTING
* 65.4 ± 0.3 Ma, Ar-Ar plateau age, Kent et al. (2002)
** 1475 ± 63 Ma, whole rock Rb-Sr isochron age, Ray Barman et al.
(1994)
*** 1550 ± 2 Ma, U-Pb zircon age by ID-TIMS, Chatterjee et al. (2007)
The CGC is primarily composed of high-grade
amphibolite-granulite facies basement gneisses and
migmatite with enclaves of supracrustal metasediments
intruded by mafic-ultramafic, massif anorthosite and granitic
rocks (Ghose et al.1973; Ghose, 1983, 1992; Ghose and
Mukherjee, 2000). The basement rocks exhibit polyphase
deformation, metamorphism and partial melting, and record
tectono-thermal events at 1.7-1.6 Ga and 0.9-1.1 Ga (Mallik
et al. 1991; Ray Barman and Bishui, 1994; Singh et al. 2001;
Maji et al. 2007). An older pre-1.6 Ga fabric is evident in
sillimanite-bearing clasts of basal conglomerate (Ghose and
Mukherjee, 2000) in the Bihar Mica Belt (Fig. 1B), a
metamorphosed cover sequence in northern CGC intruded
by granite at 1590 ± 30 Ma (Pandey et al. 1986). The Bengal
anorthosite was emplaced at 1550 ± 2 Ma (U-Pb zircon
age, Chatterjee et al. 2007). Younger magmatic events
include Rajmahal basaltic volcanism at 115-118 Ma (Baksi,
1995; Kent et al. 2002) and Salma mafic dyke intrusion at
65.4 ± 0.3 Ma (Kent et al. 2002) (Fig.1C). Based on available
geochronological data and mutual relationship of rocks, a
simplified stratigraphic sequence of the eastern part of the
CGC is presented in Table 1.
In the vicinity of the Bengal anorthosite (Fig. 1), the
basement rocks include quartzofeldspathic gneiss, biotite
(bt)-hornblende (hbl) gneiss, hbl gneiss, migmatite,
charnockite and leptynite, and the supracrustal metasediments include calc-silicate granulite, quartzite, mica
Age
Lithology
Recent
Early Tertiary
Alluvium
Salma dyke (dolerite)*
Permo-Carboniferous
(Lower Gondwana)
Sandstone with shale partings and coal seams
with intrusive dolerite, lamproite and
lamprophyric rocks
- - - - - - - - - - - - - - - - - -Unconformity - - - - - - - - - - - - - - - - - - - Pegmatite, aplite and quartz vein
Alkali granite (hypersolvus)
Biotite granite
Mesoproterozoic
(1600-1400 Ma)
Syenite and nepheline syenite**
Bengal Anorthosite massif***
Basic granulite, amphibolite and hornblendite
Paleoproterozoic
(>1600 Ma)
Supracrustal enclaves of metapelite (mica
schist, sillimanite schists and khondalite),
meta-arenite (quartzite and quartz-magnetite
rock) and calc-silicate gneiss in basement
complex comprised of quartzofeldspathic
gneiss, migmatite, augen gneiss, biotite/
hornblende gneiss, charnockite, and leptynite
schist, khondalite, sillimanite schist and quartz (qtz)magnetite (mag) rock. The anorthosite and subsequently
granites intruded the granulite to amphibolite facies country
gneisses and metasediments. Several bands of metabasic
rocks (mafic granulite and amphibolite) containing
plagioclase (pl), hbl, clinopyroxene (cpx)±orthopyroxene
(opx)±garnet (grt) occur both inside and outside the massif
(Ghose et al. 2005). They are parallel to the E-W regional
trend (Fig. 1C) and show branching and pinch-and-swell
structures inside the massif. Folded veins and tongues of
anorthosite occur within the metabasic rocks (Mukherjee,
1995) and metabasic xenoliths occur within the anorthosite
(Fig. 2A) indicating that the anorthosite and metabasic suites
are coeval. Previous thermobarometric calculations
indicated similar pressures and temperatures (P-T) for the
Bengal anorthosite, metabasites and the country rocks
(4.0-8.5 kbar/600-830 oC, Table 2, Fig. 3). Sen and
Bhattacharya (1993) interpreted ~7.5 kbar and ~830 oC as
conditions slightly below that of peak metamorphism.
Recent work has attempted to correlate P-T’s (Maji et al.
2007) to metamorphism associated with the three
recognizable deformations (D 1, D2 and D3) in the area
(Roy, 1977; Mukherjee, 1995).
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MINERALOGY AND GEOCHEMISTRY OF BENGAL ANORTHOSITE IN CHOTANAGPUR GNEISSIC COMPLEX
265
Fig.1. (A) Location map of the Bengal anorthosite at Saltora (solid circle), Eastern India. EGB - Eastern Ghats Belt, SC - Singhbhum
craton, CGC - Chotanagpur Gneissic Complex, CITZ - Central Indian Tectonic Zone, dashed line - Singhbhum shear zone,
stippled area - Gondwana Basins: M-Mahanadi Basin, G-Godavari Basin. (B) A simplified map of the CGC showing location of
the Bengal anorthosite at Saltora and other anorthosite occurrences (solid circle), Gondwana Basins along the Damodar Graben
(stippled area) and Proterozoic Basins (inclined bars). (C) A detailed map of the Bengal anorthosite and its surrounding areas
(modified after Roy, 1977; Bhattacharyya and Mukherjee, 1987) and an orientation plot for primary flow structures in anorthosite
(after Mukherjee, 1995).
In and around Saltora, the poorly preserved earliest nondiastrophic sedimentary stratification (S0) was deformed
during D1 to tightly appressed, isoclinal, rootless, intrafolial
folds with E-W northerly-dipping axial planes (S1). The S1
fabric is characterized by inclusions of pl+ilmenite (ilm)±hbl
in opx and cpx in mafic granulites, grt+opx in pl-bearing
leucocratic and bt±hbl melanocratic layers in migmatitic
gneisses, cpx+scapolite (scp)+pl+K-feldspar (kfs)+calcite
(cc)+qtz in calc-silicate gneisses, and grt+sillimanite
(sil)+kfs+qtz+spinel (spl)+ilm in metapelites (Maji et al.
JOUR.GEOL.SOC.INDIA, VOL.72, AUGUST 2008
2007). These authors further suggest that inclusions of bt,
pl, kfs and ilm in gt indicate a prograde formation of garnet
accompanied by melting at 750–850ºC and 4-6 kbar. The
Bengal anorthosite lacks the S 1 granulitic fabric, but
contains the subsequent fabrics. Hence, it is post-D1/pre-D2
and was emplaced into D1-deformed high-grade gneisses
(Maji et al. 2007) at 1.55 Ga (Chatterjee et al. 2007).
The D2 episode produced asymmetric and overturned
folds coaxial with the first generation folds. S1 was
isoclinally folded during D 2 with development of a
266
NARESH C. GHOSE AND OTHERS
Table 2. Equilibrium pressures and temperatures of the anorthosites and associated rocks
ToC
(min)a
ToC
(max)a
P kbar
(min)a
P kbar
(max)a
Formulationb
Equilibria
Referencec
Massif
Amphibolite/Meta-norite
761
807
5
hbl-plag
15
C
Amphibolite/Meta-norite
Basic granulite
619
768
670
794
6
6.3
two-px
two-px
18
16
C
A
Basic granulite
Basic granulite
619
604
665
659
6.3
6.3
opx-gar
cpx-gar
14
7
A
A
Basic granulite
Basic granulite
711
650
761
750
6.3
5.7
cpx-gar
px-gar-plag-qtz
9
21
A
A
Amphibolite/Meta-norite
Gabbroic anorthosite
761
704
807
736
6.4
5
15, 1
15
C
C
Gabbroic anorthosite
Gabbroic anorthosite
729
625
757
658
5
6
hbl-plag
two-px
15
18
C
H
Gabbroic anorthosite
Gabbroic anorthosite
623
641
gar-bt
gar-hbl
8
12
H
H
Gabbroic anorthosited
Gabbroic anorthosited
616
593
672
649
4.3
4.3
4.7
4.7
cpx-gar, cpx-gar-plag-qtz
cpx-gar, cpx-gar-plag-qtz
7, 21
24, 21
H
H
Gabbroic anorthosite
Gabbroic anorthosite
616
704
672
736
4.1
5.6
4.4
6.1
cpx-gar, cpx-gar-plag-qtz
hbl-plag, Al-in-hbl
7, 21, 6
15, 1
H
C
White anorthosite
Mottled anorthosite
669
726
795
5
5
hbl-plag
hbl-plag
15
15
H
H
Anorthosite
Anorthosite
720
670
7
7
opx-gar
opx-gar
25
14
E
E
Anorthosite
White anorthosite
645
724
715
6
opx-gar/cpx-gar
gar-bt
14, 25, 7
8
E
H
White anorthosite
Anorthosite
646
600
800
6.6
7.4
gar-hbl
opx-gar-plag-qtz
12
21
H
E
White anorthosite
683
696
5.6
7.3
hbl-plag, Al-in-hbl
15, 1
H
6.6
hbl-plag, Al-in-hbl
hbl-plag
Country Rocks
Meta-norite
778
800
5
hbl-plag
15
C
Amphibolite
Meta-norite
667
638
724
659
5
6
hbl-plag
two-px
15
18
H, B
C
Basic granulite
Basic granulite
620
600
720
630
7
6
opx-gar
opx-gar
25
14, 3
G
F
Basic granulite
Basic granulite
660
720
700
740
6
6
opx-gar
opx-gar
25
17, 3
F
F
Basic granulite
Basic granulite
650
650
720
700
7
6
cpx-gar
cpx-gar
7
7, 3
G
F
Basic granulite
Basic granulite
650
650
690
690
6.6
6.5
7.4
7.5
Fe-opx-gar-plag-qtz
Fe-opx-gar-plag-qtz
22
19
F
F
Basic granulite
Khondalite
650
742
690
5.7
6.3
7.1
Fe-cpx-gar-plag-qtz
cpx-gar
19
9
F
A
Metapelite
Metapelite
645
630
715
700
6
7
opx-gar/cpx-gar
gar-bt
14, 25, 7
8
E
E
Khondalite
Metapelite
650
600
750
800
5.9
4
6.5
5.4
gar-plag-qtz-sill
gar-plag-qtz-sill
20
10, 20
A
E
Basic granulite
Calc-silicate gneiss
750
820
830
840
6
7.3
8.5
7.6
hbl-px-plag-gar
plag-calc-scap-gar
4, 13, 5
11, 23, 2
D
F
a
Italicized T and P values are assumed values
These references are available from the authors upon request: 1 - Anderson and Smith (1995), 2 - Berman (1988), 3 - Bhattacharya et al. (1990), 4 - Binns (1969), 5 - Brown
(1970), 6 - Eckert et al. (1991), 7 - Ellis and Green (1979), 8 - Ferry and Spear (1978), 9 - Ganguly (1979), 10 - Ganguly and Saxena (1984), 11 - Goldsmith and Newton
(1977), 12 - Graham and Powell (1984), 13 - Green and Ringwood (1967), 14 - Harley (1984), 15 - Holland and Blundy (1994), 16 - Kretz (1982), 17 - Lee and Ganguly
(1988), 18 - Lindsley (1983), 19 - Moecher et al. (1988), 20 - Newton and Haselton (1981), 21 - Newton and Perkins (1982), 22 – Perkins and Chipera (1985), 23 - Powell
(1978), 24 - Powell (1985), 25 - Sen and Bhattacharya (1984)
c
A - Bhattacharyya and Mukherjee (1987), B - Das and Bhattacharyya (1999), C - Ghose et al. (2005), D - Manna and Sen (1974), E - Sen and Bhattacharya (1985), F - Sen
and Bhattacharya (1993), G - Sen and Manna (1976), H - This study
d
calculated from cores (higher P,T) and rims (lower P,T) of the coexisting minerals
b
JOUR.GEOL.SOC.INDIA, VOL.72, AUGUST 2008
MINERALOGY AND GEOCHEMISTRY OF BENGAL ANORTHOSITE IN CHOTANAGPUR GNEISSIC COMPLEX
267
Fig.2. (A) Metabasic xenolith within anorthosite north of Nandanpur showing diffuse contact, the hammer is placed on the metabasic.
(B) Superposition of upright F3 fold (axial trace upward in photo) on the limb of isoclinal F2 fold (axial trace side-to-side in
photo, see pencil for scale) of basement gneiss, xenolith within anorthosite south of Sitaldanga. (C) Back-scattered electron
image showing plagioclase with huttenlocher intergrowth (h-Plag) growing between amphibole (Amph) + biotite (Bt) and
K-feldspar (Kfs). (D) Details of the huttenlocher intergrowth in plagioclase. (E) Crossed polarized light image showing
garnet (Gt) corona around clinopyroxene (Cpx), amphibole and biotite surrounded by plagioclase. (F) Plane light image showing
garnet corona around plagioclase (width ~2 mm) with an outer rim of Fe-Ti oxide in white anorthosite from a borehole sample
at Nandanpur.
JOUR.GEOL.SOC.INDIA, VOL.72, AUGUST 2008
268
NARESH C. GHOSE AND OTHERS
Fig.3. Calculated equilibrium pressures and temperatures of
anorthosite, amphibolite, basic granulite, metapelite and
calc-granulite from the Bengal anorthosite massif and the
surrounding country rocks. The arrow indicates an
approximate retrograde metamorphic pressure-temperature
trajectory. Data include earlier works of Bhattacharyya
and Mukherjee (1987), Ghose et al. (2005), Manna and
Sen (1974), Sen and Bhattacharya (1985, 1993) and Sen
and Manna (1976) (see Table 2) for comparison.
penetrative foliation (S2) defined by the preferred alignment
of hbl in mafic granulite, bt in quartzofeldspathic gneisses,
and bt+sil aggregates in metapelites (Mukherjee, 1995; Maji
et al. 2007). D3 resulted in a set of east-west north-dipping
asymmetric, upright non-cylindrical shear related folds
(Fig. 2B). The expansive granites were deformed during D3
under upper amphibolite facies conditions at 1.4 -1.1 Ga
(Maji et al. 2007). The S2 and S3 fabrics are characterized
by bt±sil in gneiss and metapelite that formed at the
expense of grt, and hbl in mafic granulite that formed at the
expense of opx+cpx+pl. In the anorthosite massif, S2 and
S3 consist of hbl+pl and shape-preferred bt. The intensity of
deformation decreases inward in the massif. In the interior
part, deformation is limited to weak wavy extinction, mildly
bent twin lamellae and local bulges at grain boundaries in
plagioclase. In the peripheral parts, internally strained
plagioclase forms a fine-grained mosaic including coarsegrained elliptical plagioclase of magmatic origin with subgrain boundaries.
Post-D3 Grenvillian metamorphism at 0.9-1.0 Ga (Maji
et al. 2007; Chatterjee et al. 2007) was marked by prograde
growth of coronal and discrete grt (+qtz) at opx-pl interface
in mafic granulite and gneiss at the expense of bt+pl, and
opx-pl and hbl-pl interfaces in anorthosite at the expense of
opx and cpx at 650 ± 50ºC and 4.5 ± 0.5 kbar.
The core of the Bengal anorthosite is composed of grey
anorthosite (colour index ‹8) containing coarse-grained,
bluish grey cumulus plagioclase megacrysts showing
magmatic flow-related alignment (Mukherjee et al. 2005).
Anorthosite dykes also showing magmatic alignment of
euhedral plagioclase occur near the core of the massif
(Maji et al. 2007). The plagioclase megacrysts are poikilitic
with small inclusions of ilm, mag and bt. The periphery of
the massif is dominated by medium grained, equigranular
white anorthosite (colour index =8). The texture gradually
changes from panidiomorphic at the core to saccharoidal
near the periphery as the proportion of white granular
plagioclase increases. The zone between the grey and the
white anorthosites has a mottled appearance due to variable
amounts of the grey and white plagioclase. Schlierens of
opx-cpx-hbl-grt-bt-ilm give the anorthosites a banded
appearance. A few isolated pods of grey anorthosite occur
within the mottled anorthosite in the northern part of the
massif (Fig. 1C) and a small pocket of gabbroic anorthosite
occurs within the mottled anorthosite northeast of
Ledapalash. Lenses and bands of ilm, mag and spl-rich
rocks occur at the northern boundary of the massif (Bose
and Roy, 1966).
The sub-surface shape of the Bengal anorthosite is
sheet-like (Chatterjee, 1959) or an inverted cone/wedge
as inferred from gravity data (Verma et al. 1975;
Mukhopadhyay, 1987). Deep drilling up to a depth of
622.8 m from the surface at three locations within the massif
(Fig. 1C; Mukherjee, 1995; Mukherjee et al. 2005) revealed
repetitive graded layers of grey, white and mottled
anorthosites overlain by gabbroic anorthosite. The basal part
of the anorthosite pluton is essentially composed of denser
equigranular white anorthosite as revealed from the borehole
sections. Plagioclase megacrysts and mafic schlierens show
flow-related preferred orientation, and grt coexists with opx,
cpx, hbl, mag and ilm throughout the boreholes. Muscovite,
bt, chlorite, cc, scp and epidote are present in the upper and
the peripheral parts of the massif. Pockets of syenite, alkali
granite, pegmatite and aplite, and veins of quartz and calcite
are also encountered in the boreholes.
ANALYTICAL METHODS
Textures were studied with a polarizing light optical
microscope and by backscattered electron imaging with a
JEOL JXA-733 Superprobe at Massachusetts Institute of
Technology, Cambridge, U.S.A. Major element composition
of minerals were analyzed by wavelength dispersive
spectrometry with the same electron microprobe. The
accelerating voltage and beam current used were 15 kV and
10 nA, respectively. Typical counting times were 20-40
JOUR.GEOL.SOC.INDIA, VOL.72, AUGUST 2008
MINERALOGY AND GEOCHEMISTRY OF BENGAL ANORTHOSITE IN CHOTANAGPUR GNEISSIC COMPLEX
seconds and 1s standard deviations of the counts were 0.51%. The data were reduced with the CITZAF program
(Armstrong, 1995). The mineral analysis data are presented
in Table 3.
Samples of anorthosite and associated rocks were
analyzed for major and trace element concentrations with
wavelength-dispersive X-ray fluorescence spectrometry
(WD-XRF) at the Department of Geology, Leicester
University, U.K. Samples were crushed with a tungstencarbide crusher, powdered in an agate mortar and pressed
into pellets. Several international geostandards for major
(MRG-1, W-1 and GA) and trace (MRG-1, W-1, W-2 and
BR) elements were analyzed simultaneously for internal
consistency and to monitor the precision of the analytical
data. FeO was determined by titration following digestion
of sample in a mixture of analytical grade HF and
concentrated H2SO4 in a platinum crucible on hot plate. The
chemical analyses of samples are presented in Table 4. The
percentage average differences from the published values
of the monitor standards (Govindaraju,1994) are 0.3-2%
for major oxides, 3-13% for the minor oxides, 1-18% for
the incompatible elements (including light rare earths) and
0.1-25% for the compatible trace elements. These
differences are within the 2s analytical uncertainties of the
WD-XRF measurements. Four samples of anorthosite were
also analyzed by neutron activation analysis (NAA) for rareearth (REE) and some critical trace elements (Sc, Ta, Hf
and Th) at the Geological Survey of India, Pune. These data
are shown in Table 5.
PETROGRAPHY, MINERAL CHEMISTRY
AND METAMORPHISM
The Bengal anorthosite is dominated by labradorite. A
distribution diagram for 136 analyses of plagioclase, selected
from six samples of grey, white and mottled anorthosites,
shows a well-characterized peak between An57 and An58
(Table 3, Fig. 4). The grey plagioclase megacryst cores are
An55-63 in composition. They show kink twin lamellae, microfracture, peripheral granulation and contain fine inclusions
of ilmenite, iron oxide and biotite (Mukherjee et al. 2005).
The white granular plagioclase with a wider range of
composition (An51-66) forms a polygonal mosaic with sharp
boundaries and 120o “triple-point” contacts. Some show
albite- or pericline-twin lamellae and kink bands. Both
plagioclase-types show subtle compositional zoning. In some
samples, small Ca-rich plagioclase (An 69-80) grains
associated with hornblende, biotite and K-feldspar show
huttenlocher intergrowths (Figs. 2C,D). Subordinate
amounts (<8% by volume) of augite (Mg# 62) and
JOUR.GEOL.SOC.INDIA, VOL.72, AUGUST 2008
269
hornblende (Mg# 36-44, CaO 11.1-11.5 wt %) are present
in mafic clots and schlierens in the anorthosites (Table 3,
Fig. 2E). Augite contains exsolved lamellae of orthopyroxene and blebs of ilmenite. Corona forming garnet (Figs.
2E,F) is almandine-rich (Gr18Pyr13 Alm65). Biotite (Mg# 4346), K-feldspar and quartz occur in minor amounts and
magnetite, pyrite, apatite, sphene and zircon occur as
accessory minerals. Chlorite, muscovite, zoisite, epidote,
actinolite, scapolite and calcite occur as secondary alteration
products.
The gabbroic anorthosite contains 10-17% mafic
minerals by volume. It consists of plagioclase (An43-52),
clinopyroxene (Mg# 59-64) and hornblende (Mg# 41-45)
with minor amounts of orthopyroxene (Mg# 45-48), biotite
(Mg# 36-42), garnet (Gr22Pyr12Alm63), chlorite and ilmenite.
The compositions of clinopyroxene, plagioclase, hornblende
and orthopyroxene are similar to those of the metabasic
rocks inside the anorthosite massif (Ghose et al. 2005).
Accessory phases include apatite, zircon, K-feldspar,
chlorite, epidote and magnetite.
Garnet corona around clinopyroxene and/or hornblende,
and intergrowths of grt+ilm+hbl between plagioclase grains
originated through high-grade metamorphism during the
post-D3 stage (Figs. 2E,F). Biotite, plagioclase, hornblende
and K-feldspar may be related through chemical reaction
(Fig. 2C). Huttenlocher intergrowths in the high-Ca
plagioclase grains (An70-80) (Fig. 2D) probably originated
by slow cooling (Grove,1977). Rims of secondary actinolite
(Mg# 60) around clinopyroxene (Mukherjee et al. 2005)
may have formed later through hydrous alteration. A variety
of mineral equilibria including two-pyroxene (Lindsley,
1983), grt-bt (Ferry and Spear, 1978), grt-hbl (Graham and
Powell, 1984), cpx-grt-pl-qtz (Ellis and Green, 1979;
Newton and Perkins, 1982) and hbl-pl (Holland and Blundy,
1994; Anderson and Smith, 1995) were used to calculate
pressures of 4.1-7.3 kbar and temperatures of 593-795oC in
the anorthosites. These results are similar to those for the
country rocks and metabasic rocks inside and outside the
massif (Bhattacharyya and Mukherjee, 1987; Ghose et al.
2005; Maji et al. 2007) and define a common pressuretemperature path for the massif and the surrounding rocks
during post-D3 Grenvillian metamorphism (Fig. 3).
GEOCHEMISTRY
Both the earlier published (Mukherjee et al. 2005) and
the new data in this study (Table 4) indicate that the grey
anorthosite is richer in Al2O3 and K2O, and poorer in TiO2,
MgO, P2O5, Zr, light rare earth elements (LREE), Ba, Sr,
Zn and the transition elements, viz. Fe, Mn, V, Cr and Ni
270
NARESH C. GHOSE AND OTHERS
Table 3. Chemical composition of minerals in the Bengal anorthosite
Sample
No. of analyses
C11
core
5
rim
2
56.07
28.39
0.16
bdl
10.09
5.75
0.15
56.70
28.06
0.27
0.02
9.54
6.13
0.13
100.62
2.507
1.496
0.006
C19
megacryst granular
15
29
Plagioclase
C22
B30
granular huttenlocher megacryst
24
10
10
weight percent
100.82
53.74
30.21
0.04
0.01
11.93
4.67
0.10
0.02
100.74
53.06
30.40
0.04
0.01
12.12
4.49
0.09
0.07
100.29
53.44
30.00
0.08
0.01
11.89
4.68
0.09
0.12
100.30
0.483
0.499
0.008
2.527
1.474
0.010
0.001
0.456
0.529
0.007
2.411
1.597
0.002
0.001
0.574
0.406
0.006
8
4.999
0.9
48.8
50.3
8
5.004
0.7
45.9
53.4
8
4.997
0.6
58.2
41.2
2.393
1.616
0.002
0.001
0.586
0.393
0.005
0.001
8
4.998
0.5
59.4
39.9
0.1
2.410
1.595
0.003
0.001
0.575
0.409
0.005
0.002
8
5.000
0.5
58.0
41.3
0.2
C11
C19
C22
B30
rims cpx
C11
rims cpx
3
1
5
1
1
SiO2
TiO2
Al2O3
Cr2O3
FeO
MnO
MgO
CaO
Na2O
K2O
NiO
F
Cl
Total
41.84
1.92
12.22
bdl
19.83
0.13
7.80
11.54
1.15
1.59
41.29
1.37
13.73
bdl
20.48
0.37
6.60
11.42
0.78
1.69
42.29
1.19
13.30
0.03
19.07
0.20
8.27
11.12
1.39
0.87
38.72
3.29
13.87
0.02
19.82
0.17
6.34
11.44
1.30
2.65
53.43
0.16
2.28
bdl
16.17
0.17
13.82
11.92
0.16
0.12
0.01
0.12
98.16
0.02
0.50
98.25
bdl
0.17
97.90
bdl
0.72
98.34
bdl
bdl
98.23
Si
Ti
Al
Cr
Fe
Mn
Mg
Ca
Na
K
Ni
F
Cl
O
SCation
Mg#c
En or Pyrb
Fs or Almb
Wo or Grob
Speb
6.367
0.220
2.193
6.301
0.157
2.470
2.614
0.047
1.501
1.867
0.229
0.329
5.980
0.382
2.525
0.003
2.560
0.022
1.458
1.893
0.388
0.523
7.758
0.018
0.390
2.524
0.017
1.771
1.881
0.340
0.308
6.385
0.136
2.366
0.003
2.408
0.025
1.862
1.799
0.408
0.168
0.006
0.030
22.964
15.658
41.2
0.011
0.130
22.720
15.657
36.5
SiO2
Al2O3
FeO
MgO
CaO
Na2O
K2O
BaO
Total
Si
Al
Fe
Mg
Ca
Na
K
Ba
O
SCation
Ora
Ana
Aba
Csa
hornblende
Sample
No. of analyses
a
c
53.16
30.04
0.09
0.01
11.94
4.62
0.18
53.30
30.07
0.11
bdl
11.90
4.84
0.18
53.69
29.73
0.07
bdl
11.70
4.94
0.15
53.00
30.41
0.09
bdl
12.37
4.46
0.17
53.41
30.18
0.10
bdl
12.10
4.52
0.17
100.46
atoms
2.249
1.755
0.003
100.03
100.38
100.29
100.51
100.50
2.404
1.598
0.004
2.421
1.580
0.003
2.388
1.615
0.003
2.404
1.601
0.004
0.736
0.258
0.002
2.404
1.601
0.003
0.001
0.579
0.405
0.010
0.575
0.423
0.010
0.565
0.432
0.009
0.597
0.390
0.010
0.584
0.395
0.010
8
5.003
0.2
73.9
25.9
8
5.003
1.0
58.2
40.7
8
5.014
1.0
57.0
42.0
8
5.010
0.9
56.2
42.9
8
5.004
1.0
59.9
39.1
8
4.998
1.0
59.0
39.9
augite
opx
C11
B30
Or - orthoclase, An - anorthite, Ab - albite, Cs - Celsian;
Mg# = 100[Mg/(Mg+Fe)]; bdl: below detection limit
23
15.062
60.4
99.98
100.57
atoms
1.967
1.973
0.003
0.004
0.059
0.065
99.21
0.405
0.006
0.629
0.920
0.022
1.979
0.001
0.037
0.000
1.045
0.014
0.902
0.022
0.000
b
6
4.011
60.8
32.2
20.7
47.1
biotite
0.399
0.011
0.632
0.891
0.031
K-feldspar
C11
C19
1
1
64.18
19.32
0.28
0.01
0.05
0.70
14.41
2.31
101.27
64.13
19.53
0.04
bdl
0.08
0.57
14.91
2.73
101.99
2.958
1.050
0.011
0.001
0.002
0.063
0.847
0.042
8
4.973
88.8
0.3
6.6
4.4
2.948
1.058
0.001
garnet
ilmenite
C19
C22
C11
C22
C11
C22
5
1
4
15
10
2
2
35.79
4.22
15.53
0.01
23.14
0.06
8.28
0.07
0.07
8.79
36.08
3.38
16.83
0.04
23.05
0.09
8.22
0.06
0.14
8.95
35.08
3.91
15.57
bdl
21.43
0.08
9.89
0.12
0.12
8.19
37.58
0.03
22.00
0.01
29.25
2.05
3.28
6.29
0.01
0.00
51.54
0.03
0.02
47.35
0.33
0.30
0.02
0.00
51.79
0.08
bdl
46.66
0.67
0.27
0.03
bdl
0.05
0.08
0.14
96.19
0.13
0.41
97.38
0.04
0.19
94.62
5.501
0.488
2.813
0.001
2.974
0.007
1.896
0.012
0.020
1.724
5.718
0.403
3.145
0.005
3.055
0.012
1.942
0.010
0.042
1.810
5.435
0.456
2.843
0.039
0.038
21.923
15.514
38.9
0.066
0.111
22.646
16.318
38.9
0.018
0.051
21.931
15.549
45.1
37.85
0.06
21.95
0.01
28.79
1.38
3.03
8.03
0.01
2.777
0.011
2.285
0.020
0.036
1.618
101.11
100.48
99.57
99.56
2.969
0.004
2.029
0.000
1.889
0.091
0.354
0.675
0.001
2.968
0.002
2.048
0.001
1.932
0.137
0.386
0.532
0.001
0.000
0.986
0.001
0.000
1.007
0.007
0.011
0.000
0.000
0.989
0.002
0.001
6
4.006
61.3
32.9
20.8
46.3
6
4.001
46.3
45.8
53.1
1.1
0.004
0.051
0.874
0.049
8
4.986
89.3
0.4
5.2
5.0
C11
0.02
0.001
0.189
22.622
15.924
36.3
C11
6
2
4
weight percent
51.68
52.28
50.28
0.11
0.15
0.04
1.31
1.47
0.79
bdl
bdl
0.01
12.73
12.64
31.76
0.19
0.35
0.43
11.08
11.24
15.37
22.57
22.03
0.53
0.30
0.42
0.02
0.044
22.956
15.604
43.6
B36
megacryst granular
10
9
49.52
32.79
0.07
bdl
15.12
2.92
0.04
actinolite
1.964
0.021
2.991
1.854
0.044
0.023
B10
megacryst granular
4
11
0.991
0.014
0.010
0.001
0.001
12
8.013
15.8
11.8
62.8
22.4
3.0
12
8.006
16.6
12.9
64.7
17.8
4.6
3
2.013
3
2.009
En - enstatite, Fs - ferrosilite, Wo - wollastonite, Pyr - pyrope, Alm - almandine, Gro - grossular, Spe - spessartine
JOUR.GEOL.SOC.INDIA, VOL.72, AUGUST 2008
JOUR.GEOL.SOC.INDIA, VOL.72, AUGUST 2008
0.02
3.46
0.63
5.52
1.50
79.0
22.8
34.8
304.7
4.3
47.7
6.0
42.3
108.6
24.3
13.2
947
0.7
14.3
21.5
40.8
665
45.73
3.49
14.50
3.44
9.80
0.18
4.52
10.74
2.60
1.04
2.32
98.36
12.90
38.5
1
B12
2.65
0.9
22.9
3.4
5.3
2.0
10.1
6.7
7.5
3.1
996
4.0
604
51.08
0.11
28.47
0.28
0.55
0.01
0.53
12.42
3.65
0.84
0.04
97.98
0.80
54.1
2
B30
gabbroic
3.81
0.69
5.49
1.24
1.05
0.03
13.6
829
0.8
12.0
14.8
27.1
658
14.2
65.8
17.3
28.2
222.4
18.3
40.8
11.7
24.8
87.9
24.1
45.76
2.52
16.48
2.70
8.03
0.15
4.44
10.63
2.72
1.18
1.25
95.61
10.46
43.1
avg(5)
4.21
0.32
13.20
3.87
3.63
6.8
25.2
4.58
0.86
5.30
2.40
2.85
4.7
24.2
4.3
0.2
2.9
6.8
4.3
950
2.4
15.1
3.3
10.6
14.6
1.5
5.8
5.7
925
1.6
19.8
4.7
13.8
13.8
3.0
0.3
4.5
480
50.93
0.24
28.40
0.39
0.50
0.01
0.37
12.38
3.68
0.73
0.07
97.66
0.85
43.9
avg(3)
5.7
423
51.02
0.19
28.23
0.38
0.44
0.01
0.32
12.34
3.68
0.71
0.09
97.41
0.78
42.2
3
B36
grey
0.58
8.00
1.13
7.06
3.41
8.0
23.9
5.6
0.1
12.0
1.5
9.8
18.5
11.3
817
2.5
1.7
5.8
4.3
908
50.75
0.15
27.69
0.53
0.44
0.01
0.43
11.80
4.19
0.95
0.07
97.01
0.92
45.5
1
B10
1.18
2.19
3.1
24.5
8.0
12.7
916
6.8
5.9
2.7
12.0
7.8
5.6
6.8
1.3
381
48.82
0.13
28.15
0.12
1.03
0.01
0.22
12.60
3.53
0.60
0.24
95.45
1.14
25.6
4
B37
4.18
0.83
5.03
1.19
1.37
0.17
27.8
1.6
21.7
75.3
24.5
12.7
764
4.4
10.3
12.3
26.0
678
9.0
51.8
12.4
31.9
148.5
1
B8
mottled
4.47
1.22
3.65
1.08
1.42
0.16
ppm ratio
6.1
544
1.9
6.9
7.5
12.4
974
5.2
25.2
5.6
16.2
74.8
4.5
12.7
8.1
10.0
23.4
25.0
ppm
49.25
0.98
25.54
1.03
2.09
0.03
1.36
11.66
3.77
0.68
0.13
96.40
3.01
44.5
1
B1
2.97
0.67
4.45
3.86
2.07
0.08
4.4
25.9
3.2
5.0
381
0.5
2.2
8.5
6.2
1487
4.1
9.8
3.3
15.1
1.5
48.67
0.16
29.06
0.61
0.67
0.02
0.42
13.47
3.09
0.46
0.07
96.70
1.22
38.1
weight percent
avg(8)
6.87
1.57
4.39
1.61
19.2
23.8
15.8
2.3
5.6
49.0
8.9
11.1
4.4
3.6
5.8
4.0
1246
11.8
489
49.26
1.21
24.02
0.85
2.14
0.05
1.89
10.37
4.02
0.72
0.09
94.62
2.90
53.7
5
B23
9.06
0.75
12.08
2.49
9.70
0.12
19.5
27.5
6.4
898
0.6
3.9
9.7
5.1
1214
1.0
47.1
5.2
6.9
27.3
15.0
6.3
0.5
1
C27
white
Table 4. Chemical composition of the Bengal anorthosites
4.92
0.71
6.94
4.35
11.8
2.4
13.9
39.1
25.9
9.9
5.0
17.4
10.0
23.2
1106
4.9
420
1.3
1.7
7.4
48.93
0.49
26.45
0.93
1.68
0.03
1.20
12.08
3.60
0.56
0.20
96.15
2.52
45.9
6
B21
4.97
0.78
6.38
2.37
1.65
0.06
4.7
429
0.7
3.4
8.1
11.6
1158
4.9
21.8
4.4
13.2
46.0
15.3
10.2
6.1
9.4
14.1
24.6
49.20
0.65
25.64
0.98
1.74
0.04
1.76
12.23
3.51
0.55
0.14
96.35
2.63
54.4
avg(9)
BenAn
4.35
0.89
4.87
1.56
1.55
0.08
6.4
532
1.1
5.8
9.1
14.2
995
5.9
28.2
6.5
16.9
76.3
13.1
16.4
7.1
13.5
26.4
24.6
48.95
0.98
24.60
1.17
2.68
0.05
1.84
11.82
3.49
0.72
0.30
96.49
3.74
46.7
avg(25)
3.21
0.30
10.76
1.22
0.78
0.08
11.3
128
2.0
9.6
11.8
24.0
146
15.1
103.7
32.3
47.9
333.7
123.2
58.3
57.8
80.5
101.5
18.7
47.28
1.55
12.39
2.85
11.08
0.21
5.79
10.41
2.04
0.43
0.22
93.99
13.65
43.1
avg(5)
Lti-meta
9.66
1.66
5.83
0.77
0.96
0.05
31
350
4
48
37
80
660
38.5
280
29
OIB
8.64
1.14
7.60
1.20
1.15
0.17
112
550
10.7
25
30
64
350
26
190
22
UCC
3.68
0.32
11.67
1.83
0.87
0.05
104
2080
28
5.3
150
1.1
6
11
23
230
12.7
70
19
14.9
LCC
All concentrations except FeO determined by wavelength dispersive X-ray fluorescence spectrometry; FeO determined by wet chemistry;a avg: Average with number of analyses in parentheses (includes data from Mukherjee
et al. 2005); BenAn: Bengal anorthosite; Lti-meta: Low-Ti metabasic (Ghose et al. 2005); OIB: Ocean Island Basalt (Sun and McDonough, 1989); UCC and LCC: Upper and Lower Continental Crust (Taylor and
McLennan, 1995); b 1 - NNE of Ledapalash; 2 - Nimra; 3 - Nandanpur; 4 - NW of Nandanpur; 5 - 13 km WNW of Gangajalghati; 6 - W of Shyampur; c Mg# = 100[Mg/(Mg+Fe)]
Zr/Y
Nb/Y
Zr/Nb
La/Nb
La/Nd
Th/Ce
Rb
Ba
Th
Nb
La
Ce
Sr
Nd
Zr
Y
Sc
V
Cr
Co
Ni
Cu
Zn
Ga
SiO2
TiO2
Al2O3
Fe2O3
FeO
MnO
MgO
CaO
Na2O
K2O
P2O5
Total
FeOtotal
Mg#c
Locationb
Samplea
MINERALOGY AND GEOCHEMISTRY OF BENGAL ANORTHOSITE IN CHOTANAGPUR GNEISSIC COMPLEX
271
272
NARESH C. GHOSE AND OTHERS
Fig.4. A frequency distribution of the anorthite-content of plagioclase in the Bengal anorthosite based on 136 analyses from six anorthosite
samples, performed with the electron microprobe. The data show a well constrained peak around An57.
Fig.5. Variation of (A-D) Al2O3 and (E,F) MgO with total-FeO, CaO and Na2O, and (G,H) Zr with Ce and Nb. Symbols: open squarewhite anorthosite; solid square-grey anorthosite; cross- mottled anorthosite; open diamond-gabbroic anorthosite; solid diamond:
Low-Ti metabasic intrusive; circle: High-Ti metabasic intrusive; triangle: ultramafic intrusive.
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MINERALOGY AND GEOCHEMISTRY OF BENGAL ANORTHOSITE IN CHOTANAGPUR GNEISSIC COMPLEX
Table 5. Rare-earth and high-field-strength element concentrations of selected
Bengal anorthosite samples
35SL
White
26SL
Mottled
27SL
Gabbroic
33SL
Gabbroic
Average
parts per milliona
Sc
Th
7.1
0.6
2.2
0.1
39
0.7
8.7
0.3
14.3
0.4
Ta
Hf
0.65
0.9
0.17
0.37
1.4
2.5
0.15
0.54
0.59
1.08
La
Ce
5.6
20
7.8
10
13
20
12
18
9.6
17.0
Sm
Eu
1.92
1.7
0.53
1.9
2.87
1.8
1.74
2.4
1.77
2.0
Tb
Yb
Lu
0.13
0.12
0.12
0.05
0.17
0.03
0.22
0.59
0.2
0.14
0.39
0.03
0.14
0.32
0.10
a
All concentrations determined by Neutron Activation Analysis
compared to white anorthosite. However, all the varieties
of anorthosite plot on the same trends in inter-element
variation diagrams indicating their common origin (Fig. 5).
MgO and FeO are negatively correlated, and CaO and Na2O
are positively correlated with Al2O3 (Figs. 5A-D). Na2O is
positively correlated, whereas, CaO does not vary
significantly with MgO (Figs. 5E-F). Nb and Ce are
positively correlated with Zr (Figs. 5G-H). The average trace
element pattern of the Bengal anorthosite is similar to that
of the Bolangir anorthosite of the Eastern Ghats
(Bhattacharya et al. 1998) but the contents are higher in
the Bengal anorthosite. Both the Bengal and the Bolangir
anorthosites show high positive Sr and Eu anomalies
reflecting their feldspar cumulate nature.
Similar average Zr/Nb, Zr/Hf and Th/Ce ratios of the
Bengal anorthosite and oceanic island basalt (Table 4, Hf
273
from Table 5, Zr/Hf=34.6) possibly indicates derivation of
the anorthosites from a mantle source. Prolonged interaction
of the parental melts with the lower crust may be the reason
for similar average Zr/Y, La/Nb and Th/Ce ratios of the
anorthosites and lower continental crust (Table 4), whereas,
interaction with the upper crust may have resulted in an
enriched LILE pattern of the anorthosite (Fig. 6). Upper
crustal anatexis may have happened through heat from the
anorthositic magma. Anatectic leptynite and alkali granite
with trace element patterns similar to that of a partial melt
of typical upper crustal garnet-biotite country gneiss are
present in the vicinity of the anorthosite (Ghose and
Chatterjee, 2007). Upper crustal contamination is also
indicated by similar trace element patterns of the anorthosites
and syenite and high-Ti metabasic sample from a borehole
in the anorthosite (Ghose et al. 2005; Ghose and Chatterjee,
2007).
The associated metabasic rocks are Fe- and LILE-rich
tholeiites that can be related by fractional crystallization of
olivine, augite and plagioclase (Ghose et al. 2005). In interelement variation diagrams, the metabasic rocks plot on the
same trends as the anorthosites (Fig.5), suggesting that the
two groups may be genetically related. However, the average
Zr/Nb and La/Nd ratios of the two groups are different
(Table 4). The metabasic rocks have a lower crustal signature
as indicated by a similarity of their average Zr/Y, Nb/Y and
Zr/Nb ratios to the corresponding ratios of the lower
continental crust (Table 4). The gabbroic anorthosites have
Zr/Nb, La/Nd, Zr/Y, Nb/Y and La/Nb ratios intermediate
between the metabasic rocks and the grey/mottled/white
anorthosites (Table 4) indicating that they may have
Fig.6. N-MORB-normalized (Sun and McDonough, 1989) incompatible element contents of the Bengal anorthosites (average of this
study and Mukherjee et al. 2005) compared with the Bolangir anorthosite of Eastern Ghats (Bhattacharya et al.1998).
JOUR.GEOL.SOC.INDIA, VOL.72, AUGUST 2008
274
NARESH C. GHOSE AND OTHERS
originated through mixing between anorthositic and
precursor of metabasic end-members. Their high-Fe and
high-incompatible element contents probably indicate
that they are differentiated products of a parental mafic
magma.
DISCUSSION
Igneous textures preserved in the Bengal anorthosite
include magmatic flow related alignment of cumulus
plagioclase in the core of the massif and alignment of
euhedral plagioclase in anorthosite dykes. Zircon cores with
magmatic oscillatory and sector zoning yielded a concordant
U-Pb crystallization age of 1550 ± 2 Ma of the anorthosite
(Chatterjee et al.2007). These original textures have been
overprinted by cooling, hydration with H2O released from
surrounding crust, and subsequent poly-metamorphism that
first stabilized hornblende and then augite, orthopyroxene
and coronal garnet and zircon overgrowth rims by
dehydration with progressive heating and loading at
0.9-1 Ga (Chatterjee et al. 2007; Maji et al. 2007). The
silicate minerals equilibrated with the changing P-T
conditions and their compositions (Table 3) reflect the
conditions (Table 2, Fig. 3) of post-D 3 Grenvillian
metamorphism (Maji et al. 2007).
The depth-profiles from a borehole in the anorthosite
show cyclic variations in lithology, major and trace element
contents, and elemental ratios indicating magma
emplacement in pulses (Mukherjee et al. 2005). Also
observed are chemical variations within each cycle, which
may be explained by flotation of the grey plagioclase
megacrysts over the anorthite-rich and relatively heavier
white granular plagioclase. Higher concentrations of
transitional major and trace elements in the white anorthosite
have been related to sinking of Fe-Ti oxides. These
geochemical and lithological characteristics are consistent
with an igneous origin of the Bengal anorthosite. The
geochemical features of the Bengal anorthosite also indicate
a mantle origin, interaction and mixing with the lower crustal
tholeiitic melts, and contamination with upper crustal
anatectic melts.
Several tectono-thermal events in the central and eastern
Indian shield have been recognized from different sectors
of the Central Indian Tectonic Zone (CITZ, e.g. Sausar
Mobile Belt, 2.1 Ga and 1.7-1.5 Ga, Acharyya, 2003;
Bhowmik et al. 2005), the Singhbhum Mobile Belt (1.681.6 Ga, Sarkar et al. 1989; Sengupta et al.1994; Roy et al.
2002), and in the Eastern Ghats Belt (EGB, 1.6 Ga, Mezger
and Cosca, 1999; Shaw et al. 1997). These events may be
related to globally recognized episodes of granulite facies
metamorphism, mantle-upwelling, crustal melting and
granite intrusion (Peucat et al.1999; Oliver and Fanning,
1997). Deformation and high-grade metamorphism in the
CGC during the Paleo- to Mesoproterozoic transition has
been advocated by earlier workers (Pandey et al.1986;
Mallik et al.1991; Ray Barman et al.1994) and interpreted
as related to collision between the northern and southern
Indian blocks both in the CGC (Sarkar, 1988) and in the
CITZ (Yedekar et al.1990; Jain et al.1991; Mishra et al.
2000). The emplacement of the Bengal anorthosite
postdating the early ca. 1.7-1.6 Ga tectono-thermal event in
the CGC by ca. 50-150 Myr is consistent with a period of
quiescence between delamination of lower lithosphere
during compactional orogenesis and post-tectonic
anorthosite emplacement required for the neutralization of
horizontal forces through plateau uplift (McLelland et al.
2004).
Structural study has shown that the Damodar Graben
formed after the formation of the penetrative fabric of the
terrain (Sarkar, 1988). However, it is not clear if the graben
is post-D1/pre-D2 or post-D2. If the graben is pre-D2, there
is a possibility that it is related to the emplacement of the
Bengal anorthosite. In such a case, the Bengal anorthosite
may have originated in an extensional setting.
Acknowledgements: Borehole samples were obtained
from a deep drilling project in the Bengal anorthosite
undertaken by the Geological Survey of India with DM as
in-charge. The neutron-activation REE analysis was carried
out at the Geochemical Laboratory, Pune, India. Major and
trace elements of rocks were analyzed by N. G. Marsh, at
the Department of Geology, Leicester University. FeO was
analyzed by Anjali Gupta at the Department of Geology,
Patna University.
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